
Shedding light on the art of timekeeping 21
(fs) mode-locked lasers. In turn, these three discoveries have played a crucial role in the
timekeeping story leading to the realization of the current optical atomic clocks. Although
present-day cesium microwave frequency standards perform at an already remarkable level
(fractional uncertainty below 1 part in 10
15
), a new approach to timekeeping based on optical
atomic transitions promises still greater improvements. According to the given definition of
Q, by using optical (ν
0
∼ 10
15
Hz) r ather than microwave (ν
0
∼ 10
10
Hz) frequencies,
optical standards should be considerably more a c c urate. Also several ...